Integrated circuit (“IC”) packages are ubiquitous in modern electronic devices. A typical integrated circuit package includes an IC die (“chip”), a lead frame and a layer of protective encapsulant. Lead frames are formed by cutting a pattern cut in a thin sheet of conductive material such as copper. Lead frames typically come in strip form with many identical lead frame patterns provided in a grid array on the strip.
An IC die is a small block of semiconductor material such as silicon in which an electrical circuit that performs a predetermined function is provided. Dies often have contact pads on a top surface that allow the die circuit to be connected to external circuits.
Copper lead frames are subject to oxidation, which may cause welded connections thereto, such as wire bonded connections, to be subject to failure. For this reason, copper lead frames are often plated with other metals, such as silver, which prevents oxidation. When such metal plating is performed it is usually done prior to die mounting. Generally the entire lead frame is plated in a single operation such as by electro plating.
The usual method of forming IC packages includes mounting a number of identical dies on the identical lead frame portions of a plated lead frame strip. A bottom surface of each die is attached to a central portion of each lead frame, known as a die attach pad, usually by a layer (also sometimes referred to herein as “patch”) of epoxy. The epoxy, when applied to the die pad, is in a paste form. The position and the amount of epoxy applied to each die pad needs to be carefully controlled to prevent the epoxy from smearing and overflowing onto adjacent regions of the lead frame, where it may interfere with other packaging processes. After placing each die on a corresponding patch of epoxy paste, the lead frame strip is moved to a heat source such as a curing oven where the epoxy is cured.
After curing the epoxy, the dies are electrically connected to the corresponding lead frames. In a typical process, contact pads on the dies are electrically connected to predetermined regions on the corresponding lead frames by small thin wires by a process called wire bonding. A first end of each wire is typically bonded to a preselected die contact pad by a welding process known as first bond. A second end of each wire is typically bonded to a preselected portion of each lead frame by another bonding process known as second bond or wedge bond. Both of these bonding processes are well known and understood by those skilled in the art.
After wire bonding is completed the lead frame strips are moved to mold stations where a mold compound is applied that covers the dies, wire bonds and a large portion of each of the lead frames. Small end portions of each lead frame and the web that interconnects the lead frames are not coated with mold compound. The applied mold compound is heated until it cures to a solid state. The cured mold compound protects the encapsulated portion of each lead frame, the associated die and wire bond connections.
After curing of mold compound, the lead frame strip is cut apart or “singulated” to separate the strip into individual IC packages. Each IC package includes an encapsulated lead frame and die.
After singulation the circuitry of each IC package is tested, and those having circuit failures are discarded. A common cause of IC failures is broken welds between wire bonding wires and the lead frame. The development of production processes that reduces such defective IC packages is a continuing goal in the industry.